The invention relates to a device for firmly holding seismic means, especially sources of seismic energy, in a defined manner, and more particularly is especially intended for use when a plurality of seismic means are to be used in mutual cooperation.
In seismic surveys at sea one or a plurality of streamers, i.e. cables comprising seismic receiver equipment, e.g. hydrophones, as well as one or a plurality of assemblies of sources of seismic energy, are towed behind a seismic vessel. Depending on conditions of usage and parameters the of survey the sources of seismic energy may comprise a varying number of water guns, air guns, or the like. A typical source of seismic energy is an air gun comprising a cylindrical housing with one or a number of chambers. The chambers are charged with a compressed medium under high pressure and this medium is abruptly released to expand in the environment so that a forceful wavefront is formed which will oscillate more or less in the water. This discharge of energy which is repeated at frequent intervals constitutes a high load on surrounding mechanical equipment and will also influence the surrounding cooperating sources of energy unless special measures are taken or a defined mutual positioning is ensured.
These conditions entail high demands to the equipment being towed in the water, both as regards its structure and maintenance, and in seismic surveys a heavy item of expenditure and even an obstacle to carrying out a survey is replacement of equipment or adjustment of misalignments caused by the shock loads which are triggered by the sources of seismic energy.
Efforts were previously made to improve conditions as regards the mechanical structure and positioning of sources of seismic energy in a tow. In Norweigian patent application No. 893950, corresponding to U.S. Pat. No. 4,970,046, e.g. a frame means for firmly holding air guns in a determined configuration within a frame is disclosed, which means also comprises shock absorbing links or members in the suspension means for the guns to dampen impacts and shocks caused by firing of the gun. The structure which was developed, is relatively complicated and the design of the shock absorbing members also has to be adapted to the used sources, so that this equipment will not only be very expensive, but in order to be efficient it will also involve high costs for maintenance and adaptation.
In the applicant's NO patent application No. 88 5044 corresponding to U.S. Pat. No. 5,051,966 a frame structure is disclosed which is in principle of the same kind as the frame structure disclosed in Norwegian patent application No. 89 3950, i.e. a so called "cluster"--arrangement in which the air guns are to be firmly held in an accurately defined spaced relationship to cooperate correctly. In this structure shock absorbing members absorbing the most essential pulling forces in connection with firing are integrated in the holding means proper for separate guns, but this is also a permanent structure which has to be adapted to the calculated parameters.